How to connect to a specific task in a docker swarm service?

[japgamer]

Hey,

I have some problems with my Docker Swarm setup. I’ve created a docker service with a certain amount of replicas. The service itself is part of an overlay network which includes other services as well. Now I have to access a specific task (replica) of a docker service in the network.

I know that you can access a service pretty easily by just using the name of the service and the connection will than be loadbalanced. But I have to directly access a certain task. Is there something similar for that?

Thanks in advance!

[meyay (Metin Y.)]

This question only makes sense in a situation where a container should communicate with a specific task container in the same network. So I assume you are not asking about outside to specific container task communication.

By default, swarm services register a ipvs ip, which load balances automatically to the tasks. Whenever you use a service name inside a container network, the ip of the ipvs will be used.

You can bypass the ipvs loadbalancer, by using tasks.{servicename} (of course {servicename} needs to be replaced with the real service name}.

Though, you could try if you can leverage template placeholders for this:

---
version: '3.8'
services:
  myservice:
    hostname: 'myservice-{{.Task.Slot}}'
   ....

I am not sure if template placeholders can be used for hostname (they are not available for every configuration element), but it’s worth trying.
If it does, you should be able to access a specific task by its predictable hostname from a container in the same container network.

If you are curious about template placeholders:

Example usage of docker swarm template placeholders

Today I was playing around with mounting cifs shares into a container and figured I’d like to include the namespace to the mounted path. That made me play with the known swarm template variables (docker service create | Docker Documentation). I found the template command for X_SERVICE_LABEL_STACK_NAMESPACE usefull in my endover to have stack name specific mounts Maybee this information is usefull to others as well. The stack is called ‘test’. version: '3.7' services: ubuntu: …

[halhal]

Thanks for this, I can confirm using template placeholders in the hostname works for this. And the hostname doesn’t resolve from another container until the templated service is healthy, which is nice.